| Bamboo photo aging is the surface chemical and physical changes during outdoor application which experienced long term of sun light irradiation. The main changes are expressed as surface discoloration, rough, and wash away of cell wall materials which largely affect the visual appearance and even their function. Light aging is the major factor to induce outdoor used bamboo material surface discoloration, companied by thermal and humidity and other environmental factor, acclerated the surface deterioration process, the finally caused the surface broken and cracks, largely shorted their surface life. With the rapid expansion of outdoor bamboo material, light aging becomes to be the main factor to constrict the bamboo application, and brings so large number of economical damage that some company were shut down because of large number of compensation. Therefore, the mechanism of light aging and anti light aging technique has been greatly concerned by bamboo industry.The paper take bamboo as material, first pretreated by 3 widely used heat treatment in bamboo industry, then irradiated by UV 313 light to imitate the accelerated effect of UV light in sunlight with the purpose to study the microstructure, surface color, surface O and C component, lignin content and crystalline changes during the light irradiation, in the mean time, to analyze lignin degradation products and the extractions. The main conclusions were listed below by studying the physical and chemical character changes during bamboo light aging and comparaing the influence of heat treatment on light aging process.(1) After light aging, in tangential direction, parenchyma cells, pits and fiber all showed different degree of damage, crack were appeared at weak point of the parenchyma cells, and extended along pits or near pits to several ones. The area near the pits of proto xylem vessel were also showed lots of fine cracks which showed graver than that of the parenchyma cells. Between fibers, there were some cracks and pit broken too.(2) After light irradiation, light and hue decreased, red-green parameter a* and yellow-blue parameter b* increased apparently. Each color parameter changed greatly during the first 5 days, and then changed little as time extended, and reached a steady level after 14 days. Light and hue are highly linear related to each other for lighted colored bamboo, in order to simplify the UV discolor process, can use only light parameter to evaluate the color changes during photo aging process.(3) Lignin content of different parts of bamboo got from acetyl bromide method showed that outer layer of bamboo was the most 31.89%, and inner layer the least 25.66%. After light irradiation, they all decreased, outer layer decreased the least 11.32, and insider the most 24.44%.(4) After light irradiation,C1 s composition changed, C1 decreased apparently, andC2、C3、C4 increased, the oxidation state of carbon and binding energy increased gradually, and O/C decreased as light irradiation time passed by, which indicate that the content of lignin with low O/C decreased.(5) The FITR spectrum of bamboo showed 12 main absorption peaks, after light irradiation, the peaks that related to lignin aromatic absorption all decreased apparently, among which 1605cm-1, 1462cm-1, 1331cm-1 and 833cm-1 almost disappeared, by contrast, the peak at 1738cm-1 which represent unconjugated carbonyl C=O enhanced, so the unconjugated carbonyl groups increased during the process. Peaks at 1375cm-1, 1161cm-1 and 897cm-1 represent the absorption of polysaccharide such as cellulose and hemicelluloses all kept stable during light irradiation. Peak 833cm-1 which represent G-lignin almost disappeared after 7 days photo aging, but peaks of 1331cm-1 and 1242cm-1 which represent S-lignin remained some absorption even after 56 days photo aging, this can be predict that G-lignin was more sensitive than S-lignin. Each peaks changed greatly during the first 7 days light irradiation which mean that photo chemical changes had been initiated rapidly at the beginning, this conclusion was in according with XPS analysis. After light irradiation, the degree of crystalline increased a little, it can be predicated that light was not broke the crystalline area.(6) Weak polarity column gas chromatography(RTX-5MS)can successfully identif y the lignin degraded products and extractives, 31 peaks were identified. High content of lignin degradation product were mainly p-Hydroxybenzaldehyde, isovanillin, 4’-Hydroxyacetophenone, 4’-Hydroxy-3’-methoxyacetophenone, 3-Hydroxy-4-methoxybenzoic acid, 4-tert-Butylcatechol, 3,5-Dimethoxybenzoic acid, syringaldehyde, acetosyringone, syringicacid and β-Hydroxypropiovanillone, they shared the commonly character of consist of benzene ring and conjugated double bond with the benzene ring: aromatic ring a – carb onyl, C- C benzene conjugated double bonds and biphenyl structure, these structure were the main resources of photo discoloration of bamboo. Bamboo consist several ben zene ring sturcture mainly of phenolic and aldehyde product, but very small amount co mpared with after photo aging.(7) After classification and calculation according to lignin unit types, it was showed that among lignin degradation products, H type was least among the 3 type, and G type the most. Compared with the original composition(H:G:S=1:9.51:8.08), it can be found that H and G type lignin were more vulnerable than S type lignin.(8) After heat treatment, red-green parameter a* increased, yellow-blue parameter b* decreased, the color changed to red-green direction and became darker, dark colored bamboo remained more stable color when experience photo aging. Lignin content increased after heat treatment, and decreased after light irradiation, the decrease rate of heat treated bamboo was slower than untreated one. However, FTIR analysis showed, lignin absorbing peak of heat treated bamboo was the same as untreated with light aging time extended, consequently, photo chemical reaction are closely related to bamboo original composition and heat treatment can not slow down photo degradation character. |
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